Known in the art are graphite anodes employed in different electrochemical processes. Graphite anodes have certain advantages residing in the use of a readily-available electrode material, and insensitivity to shortings. At the same time, graphite anodes have a higher potential of chlorine liberation and, consequently, a higher voltage at the electrolyzer, a high rate of wear of the anode material, thus causing frequent dismounting of electrolyzer equipment to replace anode sets. Graphite anodes have substantial overall dimensions and weight resulting in unreasonably high overall dimensions of electrolyzers and working areas in electrolytic workshops.
At present, electrodes with a conducting base and an active mass deposited thereon are extensively used in the art. The current-conducting base is made of a metal passivating under anodic polarization, such as titanium, tantalum, zirconium, niobium or alloys of these metals. The current-conducting base may be of any required shape, such as a flat plate with or without perforations, as a rod, screen, grate or a metalloceramic body.
The active mass contains oxides or mixtures of oxides of metals pertaining to the group of platinum such as ruthenium, iridium; or oxides of passivating metals such as titanium and zirconium. The active mass is applied onto the base in a thin layer with a thickness of from 3 to 10 mcm. Metal-oxide anodes, as compared to graphite ones, have improved electrochemical characteristics, a lower potential of chlorine liberation, unchangeable dimensions during a long-term service period, lower overall dimensions and weight, a high stability of the active mass and a long run (several years) between replacements of the anode sets.
Also widely used in the art are metal-oxide anodes containing in their active mass RuO.sub.2 in the amount of 46% by weight and TiO.sub.2 in the amount of 54% by weight (cf. USSR Inventor's Certificate No. 369923). The rate of consumption of the active mass under stationary conditions of chlorine electrolysis at a current density of 0.2-0.4 A/cm.sup.2 is equal to 2.6.times.10.sup.-8 g/cm.sup.2.hr as determined by the radiochemical method. The method of radiochemical analysis comprises subjecting a sample with the active mass to irradiation with a flux of neutrons (1.2-3.times.10.sup.-- neutrons/cm.sup.2.sec) for a period of from 200 to 400 hours and then the radioactive isotope of ruthenium is quantitatively determined during electrolysis in a solution, slime or gas phase.
To determine the stability of the active mass, we have used the method of variable polarity and amalgamation which is extensively employed as an express-method for the control of quality of an active coating, namely its stability upon amalgamation, adherence to the current-conducting substrate, resistance against cathodic polarization and shortings.
The method of variable polarity and amalgamation resides in the following. A test sample is alternatively subjected to anodic and cathodic polarization for 40 minutes (2 minutes--anodic polarization, 2 minutes--cathodic) at the current density of 1 A/cm.sup.2, temperature of 60.degree. C. in a solution with the content of sodium chloride of 300 g/l. One cycle of tests lasts for 40 minutes. Then the anode is dipped for 30 seconds into sodium amalgam with its concentration of 0.2% by mass. After these tests the anode is washed in distilled water, dried and its weight loss is then determined.
The results of measurements of consumption of the active mass of said anode determined by the method of variable polarity and amalgamation are shown in the following Table 1.
TABLE 1 ______________________________________ Number of test cycles 1-3 4-6 7-9 10-12 13-15 16-18 ______________________________________ Consumption of active mass for every 3 test cycles, mg/cm.sup.2 0.595 0.610 0.140 0.180 0.190 0.170 ______________________________________
As is seen from the results of radiochemical tests and the results shown in Table 1 above, the anodes currently employed on a large scale in electrolysis in many countries of the world still have an insufficient resistance of the active mass against amalgamation under the conditions or mercury electrolysis; and insufficient stability, relatively high consumption rates of noble metal for the manufacture of the anodic coating. In the chlorine electrolysis with a mercury cathode, the service life of anodes does not exceed 1-2 years; under conditions of membrane electrolyzers the service life of such anodes is not longer than 4-5 years.
Known in the art is an electrode, wherein there is deposited onto a titanium current-conducting base, for the purpose of increasing the electrode stability, an oxide of a metal of the platinum group and considerable amounts of silica. For example, the active mass contains 42% by mass of RuO.sub.2 and 58% by mass of SiO.sub.2. The rate of consumption of the active mass of this anode, as determined by the method of variable polarity and amalgamation, is 0.99-1.20 mg/cm.sup.2 for 3 test cycles (cf. French Pat. No. 2,040,116 or U.S. Pat. No. 3,846,273).
Also known are electrodes with their active mass containing silica with titania or with an oxide of another passivating metal; the total content of oxides of metals of the platinum group in their active mass is above 50% (cf. British Pat. No. 1,168,558). The rate of consumption of the active mass of the anode containing (% by mass) RuO.sub.2 --53.3, SiO.sub.2 --37.5, TiO.sub.2 --9.2 determined by the method of variable polarity and amalgamation is equal, for 3 test cycles, to 0.75 mg/cm.sup.2. A substantial disadvantage of all the prior art electrodes resides in a considerable rate of consumption of noble metal. Introduction of even considerable amounts of silica in combination with an oxide of a metal of the platinum group into the active mass does not ensure an increased stability of the latter.
Also known is an electrode, wherein onto a base of a passivating metal an active mass is deposited which contains more than 50% by mass of oxides of passivating metals, as well as oxides of metals of the platinum group and an additive, i.e. silica (cf. British Pat. No. 1,463,553, Cl.C25B 11/00, issued 2.02.1977). This electrode containing in the active mass 34.8% by mass of RuO.sub.2, 61.6% by mass of TiO.sub.2 and 3.6% of SiO.sub.2 has the rate of consumption of the active mass as determined by the method of variable polarity and amalgamation of 0.7 mg/cm.sup.2 for 3 test cycles. In other words, this electrode is substantially identical to the previously described electrode in terms of stability of its active mass, though it has an advantage over this latter electrode which resides in a lower rate of consumption of noble metal for its manufacture.